Double-fed asynchronous machines in the performance range of from 20 MVA to 500 MVA or above can be used for energy production with variable rotation speed. These machines are characterized by a distributed three-phase winding on the rotor. The rotor winding comprises individual bars, which are embedded in slots in the rotor laminate stack. The individual bars are interconnected to form a winding in the end winding. The currents are fed via at least three sliprings, which are fastened to the shaft at the end of the machine. A detail of such a machine is shown in FIG. 1 in very simplified form. The asynchronous machine 10 illustrated in FIG. 1 has a machine axis 13. A central body 11 with a shaft on which the sliprings 12 are arranged is capable of rotating about this axis 13. The rotor laminate stack 14 is arranged around the central body 11, and an auxiliary rim 20 adjoins the rotor laminate stack beneath an end winding 16 of the rotor winding. The rotor laminate stack 14 is surrounded concentrically by a stator laminate stack 15, in which a stator winding is accommodated which protrudes outwards with a stator end winding 17 at the end of the stack. The rotor laminate stack 14 is illustrated in enlarged form in detail in FIG. 2.
Since the rotors of double-fed asynchronous machines bear a rotor winding 18, said rotor winding needs to be protected from the centrifugal forces occurring. The rotor laminate stack serves firstly to absorb these forces and at the same time defines the path of the magnetic flux. The auxiliary rim 20 serves to absorb the centrifugal forces which act on the rotor end winding 16. The auxiliary rim 20, as well as the rotor laminate stack 14, consist of layered laminations, which are pressed in the axial direction to form a composite. It is known to use in this case a press plate 19, which distributes the pressure power applied by bolts 21, 22 between the laminations of the rotor laminate stack (see, for example, DE-A1-195 13 457 or DE-A1-10 2007 000 668).
Various demands are placed on the rotor laminate stack 14. FIG. 2 illustrates the basic division into an electrical region 14a and a mechanical region 14b. Firstly, the intention is for sufficient axial pressure to be provided between the layers of the laminations in the teeth for guaranteeing the homogeneity of the stack. In order to avoid vibrations, the layers should not work loose since relative movements between the teeth and the rotor winding 18 could damage the insulation. Secondly, the pressure should not be too great in order to avoid damage to the insulation layers between the individual laminations since such damage would result in increased losses. The axial pressure is intended to be higher in the mechanical region 14b of the rim than in the electrical region 14a in order to obtain a certain frictional force between the laminations.